1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device including a wire loop to be formed by wire bonding, and to a structure of a wire bonding apparatus for use in the method.
2. Description of the Related Art
In semiconductor manufacturing processes, wire bonding apparatuses are often used for bonding of thin metallic wires to electrode pads on a semiconductor chip and electrode leads on a lead frame for connection therebetween. Many wire bonding apparatuses include a bonding arm configured to be rotated with a drive motor, an ultrasonic horn attached to the bonding arm, a capillary attached at one end of the ultrasonic horn, and an ultrasonic vibrator attached to the ultrasonic horn. In such wire bonding apparatuses, the bonding arm is driven and rotated to move the capillary in a direction toward and away from a pad or lead and to bond an initial ball formed at a tip end of the capillary or a wire to the pad or lead, and then the ultrasonic horn is resonated with the ultrasonic vibrator to provide ultrasonic vibration at the tip end of the capillary for bonding.
For wire connection between a pad and a lead using such a wire bonding apparatus, a method is generally employed in which a wire extending out of the tip end of the capillary is first formed into an initial ball and the initial ball is bonded onto the pad, and then the capillary is raised to loop and bond the wire onto the lead, thereafter the wire is raised together with the capillary and cut off (see FIGS. 5 and 6 of Japanese Patent No. 3830485, for example). The method for wire connection between a pad and a lead provides a sufficient strength of bonding between the pad and wire because the initial ball is bonded to the pad. However, the strength of bonding between the lead and wire can not be sufficient because only a lateral face of the wire is bonded to the lead.
To address this problem, there has been proposed a method in which after a wire is bonded to a lead, the capillary is raised toward a pad while the wire is fed out, and then the wire is re-bonded at a position farther from the pad than the position where first bonded to the lead, thereafter the wire is raised together with the capillary and cut off (see FIG. 2 of Japanese Patent No. 3830485, for example). There has also been proposed a method in which after a wire is bonded to a lead, the capillary is raised and lowered to be brought into contact with the lead at a position farther from a pad than the position where first bonded to the lead to apply ultrasonic vibration thereto (see FIG. 4 of Japanese Patent No. 4365851, for example).
However, in the related art described in Japanese Patent No. 3830485, since the wire is bonded to the lead to form a wire loop and then the capillary is moved while the wire is fed out to form a second small wire loop on the opposite side of the lead with respect to the pad, it is necessary for stable formation of the second wire loop to increase the wire feed length, so that the wire is likely to be formed into a loop shape. Thus increasing the wire feed length will result in an increase in the size of the second wire loop and thereby require the lead to have an increased area for bonding, suffering from a problem of increase in the size of the semiconductor device. Also, in the related art described in Japanese Patent No. 4365851, since the capillary is brought into contact with the lead at a position farther from a pad than the position where first bonded to the lead, it is necessary for the lead to have an increased length, suffering from a problem of inadequacy in manufacturing a compact semiconductor device.